This invention relates to an irreversible semiconductor switching element and a semiconductor memory device utilizing the same.
For use as a semiconductor switching element, many types of semiconductor elements such as a diode utilizing a PN junction, a transistor utilizing a PNP or NPN junction, a MOS transistor or thyristor have been developed. In each of these known semiconductor switching elements, the state of the element is changed reversibly by changing such parameters as the internal conditions or the external conditions of the element so that when the switching state of the element is specified at a certain state, it is necessary to adjust the bias in order to set the specific state. For the purpose of solving this problem it has been proposed to predetermine the relative connection of the component elements at the time of assembling the same such that as if the switching element were arranged to adjust the bias. According to this method, however, it is necessary to forecast the state of the location at which the switching element is to be located at the time of designing. Such method requires a complicated design. Further, when such circuit is incorporated into an integrated circuit it is necessary to design the circuit to meet the requirement of the user thereby increasing the cost of the integrated circuit and the cost of apparatus utilizing the same. A typical example of the electronic apparatus utilizing such semiconductor switching element is a read only memory device (ROM). Although various types of ROM are not available, socalled PROM, that is read only memories that can be written are generally used. More particularly, typical PROMs include a fusible-link PROM, a blown junction type PROM and a nonvolatile MOS PROM etc.
In the fusible link PROM, a fuse made of aluminum, a nickelchromium alloy or polycrystalline silicon is incorporated into each memory cell of an IC ROM and writing is performed by passing current through the fuse of a specific memory cell selected according to a condition desired by a user and blowing the fuse. In the blown junction type PROM the fuse is replaced by a diode, and writing is performed by destroying the diode characteristics by passing large current through the diode. With PROMs of this type it is necessary to pass large current of the order of from 30 to 200 mA to write. But this is not desirable because the ROM is constituted by an integrated circuit. Especially, in the former type, the blown fuse spatters so that it is necessary to provide a cover for the fuse or to limit the risetime of the blowing current for the purpose of preventing spattering of the blown fuse.
In the nonvolatile MOS PROM a modified construction of a MOS type field effect transistor is used in which electric charge is injected into the trap center of an insulator by tunnel effect or electric charge is injected into a floating gate by tunnel effect or avalanche effect thereby storing a binary "0" or "1" according to the change in the state of the charge.
However, in these types of the PROMs the voltage for writing is relatively high of the order of 50 volts. Decrease of the write voltage is difficult due to the construction of the PROM.